In an apparent breakthrough in a protracted dispute between Tokyo and Washington, Japan has agreed to a Reagan administration proposal that will allow its nuclear reprocessing plant to operate through the end of 1984.

Japanese officials today announced their decision to go along with a plan that will lift U.S.-imposed controls on the amount of fuel that can be reprocessed at Japan's experimental plant near Tokyo and pave the way for the construction of a larger commercial reprocessing facility.

The tentative agreement is important because it should enable Japan to go ahead with its ambitious plans for nuclear development. In making its decision, however, Tokyo appears to have retreated from its earlier demands for a permanent settlement on the issue that would remove all major restrictions on reprocessing, including the time limits that had been imposed in the past at the insistence of the Carter administration.

One government official here said that the Japanese were "not entirely satisfied" with the U.S. proposal, which was submitted to Science and Technology Agency Director General Ichiro Nakagawa on Thursday by U.S. Ambassador Mike Mansfield.

But they agreed to accept it, he said, because of Tokyo's appraisal that the Reagan administration is likely to strike a much less rigid posture on the issue of nuclear nonproliferation than the Carter White House. Officials also said that it would open the door to a long-term and more liberal arrangement with the United States before the accord expires at the end of 1984.

In theory, failure to come to terms could have forced the shutdown of Japan's pilot plant at Tokaimura. It started full-scale operations in January and is legally permitted to reprocess spent nuclear fuel only until Oct. 31 under the existing agreement with the United States.

During the Carter years, the issue was one of the touchiest subjects in ties between the two countries. At first, Carter strongly objected to the Tokaimura facility because it had the potential for turning out the high-grade plutonium used in the production of nuclear weapons.

Japan hotly disputed any suggesstion that it might start manufacturing those weapons and argued that the experimental plant was central to its long-term plans for the development of its nuclear power generating capacity to help cut its reliance on imported gas and oil.

In September 1977, the two countries agreed on a two-year operating plan with strict controls on the amount of spent fuel that could be reprocessed. Since then, that agreement has been extended in stages to the end of next month.

Talks between officials of the two governments became stuck earlier this year on Tokyo's insistence on getting the United States to do away with all major controls, something the United States flatly rejected, at least for the momemnt.

"The basic policy of the Reagan administration," as one Japanese official viewed it, "is still nonproliferation." He said there were signs that pointed to significant easing of the Carter administration's previous hard-line stance, but, "if it looked as if it was favoring Japan too much, it could draw fire from other countries in the Middle East and South Asia" who want to expand their own reprocessing capacities.

Nakagawa told reporters that the new agreement represented a "vast improvement" over the 1977 accord but that "it will take more time to reach a permanent solution." Disgruntled government officials, however, expressed their disappointment in light of the joint statement issued after the Japan-U.S. summit in May which, they pointed out, called for immediate negotiations on a long-term settlement.

Japan buys the enriched uranium used in its 21 nuclear reactors from the United States, which maintains the right to put restrictions on its reprocessing. Since 1977, the United States has given the go-ahead for the Tokaimura plant to reprocess 149 tons of fuel but, in fact, it has so far reprocessed only 106 tons.

In principle, the question of U.S.-imposed restrictions is vital for the future of Japan's nuclear development policies. By the end of 1980, Japan's demand for reprocessing was about 400 tons a year, forcing its electric power companies to ask Britain and France to reprocess the bulk of its used fuel.

In a bid to establish its own independent nuclear fuel cycle, Japan wants to build a large-scale plant for commercial use which it plans to put into operation in 1990 with a yearly capacity of 1,200 tons. Under the new accord with the United States, Japanese officials said, Washington has agreed to lift its earlier curbs on the construction of the plant but will retain a say in its eventual operations.